The present invention relates to a hydraulic unit in which a hydraulic pump is driven by a variable-speed motor driven by an inverter.
Conventionally, there is a hydraulic unit in which a servomotor for driving a hydraulic pump is controlled in response to operation of an actuator, by a pressure control signal representing a deviation of a pressure command signal inputted from outside (main unit side) and a pressure signal representing a discharge pressure of the hydraulic pump detected by a pressure sensor at the time of pressure control while controlled by a flow rate control signal representing a deviation of a flow rate command signal inputted from outside and a rotation speed signal of the servomotor equivalent to a flow rate detected by a rotation sensor at the time of flow rate control (Japanese Patent Laid-Open Publication No. 5-196001).
However, since input signal wires are required to input the pressure command signal and flow rate command signal from outside in the conventional hydraulic unit, a problem arises that the surrounding of the hydraulic unit becomes disorderly with these input signal wires and a power wire.
Accordingly, an object of the present invention is to provide a hydraulic unit which does not require input signal wires connected from outside.
To attain the above-described object, the autonomous inverter-driven hydraulic unit of the present invention comprises:
a hydraulic pump;
a variable-speed motor for driving the hydraulic pump;
an inverter for driving the variable-speed motor;
a load sensor for detecting a load of the hydraulic pump;
a rotation sensor for detecting a rotation speed of the variable-speed motor or hydraulic pump; and
a controller for outputting a control signal to the inverter based on outputs from the load sensor and rotation sensor so that a pressure and flow rate of fluid discharged from the hydraulic pump become a pressure and flow rate on a predetermined target pressure-flow rate characteristic line.
According to the above constitution, the controller outputs the control signal to the inverter based on the outputs from the load sensor and rotation sensor without receiving a pressure command signal or flow rate command signal from outside so that a pressure and flow rate of fluid discharged from the hydraulic pump become a pressure and flow rate on the predetermined target pressure-flow rate characteristic line. Thus, this autonomous inverter-driven hydraulic unit controls the rotational number of the variable-speed motor by the controller autonomously via the inverter to control the pressure and flow rate of the fluid from the hydraulic pump without receiving a pressure command signal or flow rate command signal from outside. Therefore, input signals wire can be omitted and no human operation for inputting these command signals is required since the pressure command signal and flow rate command signal do not need to be inputted from outside. Thus, wiring is simplified.
In one embodiment, the target pressure-flow rate characteristic line is composed of a maximum flow rate line, maximum horsepower curve or about maximum horsepower line, and maximum pressure line.
According to the above constitution, for example, at the time of pressure keeping, the controller rotates the variable-speed motor via the inverter so that the hydraulic pump discharges at a low flow rate represented by a point on a maximum pressure line substantially in parallel to a flow rate axis. Therefore, since the variable-speed motor and hydraulic pump do not rotate at a rotation speed higher than necessary, energy saving can be achieved with little horsepower loss and noises can be reduced. Also, when no pressure is required, the controller rotates the variable-speed motor via the inverter so that a discharge pressure of the hydraulic pump becomes a low pressure represented by a point on a maximum flow rate line substantially in parallel to a pressure axis. Therefore, since the variable-speed motor and hydraulic pump do not rotate at a rotation speed higher than necessary, energy saving can be achieved with little horsepower loss and noises can be reduced. When a maximum horsepower is required, the controller rotates the variable-speed motor via the inverter to obtain a value on the maximum horsepower curve or pseudo maximum horsepower line.
In one embodiment, the controller has a target horsepower calculation unit calculating a target horsepower based on the outputs from the load sensor and rotation sensor and the target pressure-flow rate characteristic line, a present horsepower calculation unit calculating a present horsepower based on the outputs from the load sensor and rotation sensor and a comparison unit comparing the target horsepower and the present horsepower and outputting the control signal to the inverter.
According to the above constitution, the target horsepower calculation unit of the controller calculates the target horsepower based on the outputs from the load sensor and rotation sensor and the target pressure-flow rate characteristic line without receiving a command signal from outside. On the other hand, the present horsepower calculation unit calculates the present horsepower based on the outputs of the load sensor and rotation sensor. Then, the comparison unit compares the target horsepower and the present horsepower and outputs a control signal to the inverter so that the present horsepower becomes the target horsepower. Thus, the flow rate and pressure of the fluid discharged from the hydraulic pump easily become values on the target pressure-flow rate characteristic line.
In one embodiment, the load sensor is a current sensor for detecting current flowing to the variable-speed motor.
According to the above constitution, since the load sensor is the current sensor for detecting current which flows to the variable-speed motor, the discharge pressure of the hydraulic pump can be easily detected.
In one embodiment, the load sensor is a pressure sensor for detecting a pressure in a discharge line of the hydraulic pump.
According to the above constitution, since the load sensor is the pressure sensor for detecting the pressure in the discharge line of the hydraulic pump, the pressure in the discharge line of the hydraulic pump can be detected directly and precisely.